The invention relates generally to electromagnetic signalling and telemetry for biomedical implantable devices.
The increasing versatility of implanted stimulators such as cardiac pacers demands more complex programming capabilities. Programming in this context means noninvasively transferring parameter value data from an external device called the programmer to an internal device implanted in the patient's body. A number systems have been successfully employed in commercially available cardiac pacers, including magnetic programming and radio frequency (RF) programming. RF programming suffers from the unavoidable inherent disadvantage of electromagnetic interference in the environment which an active cardiac pacer patient may encounter. Magnetic programming, on the other hand, relies upon the generation of a series of strong magnetic impulses which actuate a reed switch inside the pacer. Additional circuitry recreates the magnetic pulse waveform from the openings and closings of the reed switch in response to the pulsating magnetic field. The output of the reed switch circuit forms the programming input to programming data registers in the implant, as shown, for example, in U.S. Pat. No. 3,805,796 to Terry et al., assigned to the assignee of the present application.
Reed switch programming is not to be confused with so called "magnet rate, " long a standard feature of demand cardiac pacers. Cardiac pacers which stimulate only when necessary to fill in "missing beats" are called standby or demand pacers. To check the inherent fixed rate of this pacer, i.e., the rate to which the pacer will revert in the absence of spontaneous cardiac activity, the physician can place a permanent magnet over the pacer site to actuate a reed switch. The pacer circuitry is designed to respond by removing the demand feature and switching to asynchronous or fixed rate operation so that the physician can not only check the fixed rate but can determine whether the pacer still is operating above the "capture threshold" and, in certain designs, check the pacer's batteries. In pacers such as Cordis "Omnicor.RTM.", where the same reed switch is used for this diagnostic mode or magnet rate, and for programming (i.e., selecting new parameter values), the pacer is equipped with timing circuitry to distinguish between magnetic impulses and a constant magnetic field.
Reed switches have a number of desirable attributes. Besides having little or no associated current drain in the quiescent mode, the insensitivity of reed switches protects against spurious programming. In addition, of course, it is possible to use the same sensor for the conventional diagnostic mode as well as for programming.
On the other hand, with the trend to smaller pacer enclosures, the size reduction of the tiny mechanical reed switch element further reduces the sensitivity of the reed switch and its reliability and range of operation. The reed switch is essentially a threshold device dependent upon the proximity of the source of the magnetic field. Thus, with a given magnetic pulse train, the duty cycle of the pulse reproduced by the reed switch circuit depends on the distance between the programmer and the implant. In addition, a debounce circuit is necessary.
Some types of pacers only count magnetic impulses, such as the Cordis Omnicor, Telectronics and the Microlith-P.RTM. manufactured by Cardiac Pacemakers, Inc. of Minnesota. However, substantial duty cycle distortion is intolerable in pacer designs using pulse width modulation of magnetic impulses.